Help- High pitch propeller in displacement boat

Discussion in 'Props' started by yodani, Aug 20, 2016.

  1. yodani
    Joined: Nov 2010
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    yodani Senior Member

    Hi there,

    I have a dilema and hope someone can help me decide what to do...

    I am in the process of repowering a boat and just bought the new engine. It is a Deutz F4L412 - four cylinder 51hp@1800 rpm air cooled.

    The boat is a 9m long with 2.8m beam at water line and 1m draft.

    I have the possibility to get a 1:2.46 Yanmar TMH50A new gearbox at a great price but the propeller diameter for this combination will not fit...

    Prop calc is recommending a 24.7x21.5 propeller but the max dia I can fit is 19 inches. By the theory I should increase the pitch to 19x32" but is that going to work? Four blade propeller...

    Any advice? Thank you.
     
  2. gonzo
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    gonzo Senior Member

    The pitch is related to the RPM. Diameter can be decreased if you increase the number of blades.
     
  3. yodani
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    yodani Senior Member

    Well a 5 or 6 blade propeller will cost more than a gearbox so ...

    I want to know how much this high pitch will influence the efficiency and the behavior like - vibration, cavitation etc.

    Was thinking about taking one propeller like this - https://www.ebay.co.uk/itm/232049420406 24x32 and reduce its diameter to 19"?

    Just a thought...

    It is hard to find a good 1:1.5 gearbox at a cheap price.... PRM 280 is reasonable but it's only 1:2, ZF 45a is the next contender but expensive. Would prefer a drop gearbox straight and nor angled but that's hard to find.

    Any recommendations?

    The Kanzaki is a nice tough gearbox rated 250hp pleasure so should be fine on thr Deutz...
     
  4. JSL
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    JSL Senior Member

    Diameter & number of blades gives you blade area-pitch does not.
     
  5. yodani
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    yodani Senior Member

    So no one knows what is likely to happen:)?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The boat will continue to move forward, but the efficiency of the thing will be poor. :D
     
  7. CatrigCat
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    CatrigCat Junior Member

    51@1800 rpm and 2.46 reduction propably need 27-29" diameter.
    If you reduce that to 19" by 32" pitch you will have 20-25% less efficiency.
    You will cruise fine (not efficiently though) but the small prop will cavitate at full power.

    Too much pitch instead of diameter makes the propeller wake to rotate and reduces efficiency.
    Some claim that a really big rudder can gain back some of the wake's rotation lost efficiency but I know nothing about that.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    :eek: Sounds like an old wive's tale, but stand by for further comment ! :D
     
  9. MikeJohns
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    MikeJohns Senior Member

    Not a myth. The loss is called rotational energy loss, anything that reduces the fluid rotation decreases that loss and recuperates some of that energy.

    Vane wheels, counter rotating props, inline rudders and pre-swirl devices are used.
     
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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Fascinating. How does the rudder recapture that energy for the forward motion of the boat ?
     
  11. MikeJohns
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    MikeJohns Senior Member

    Importantly you should run your prior setup through whatever software you are using to get some validation of the hull resistance.

    The 2.5:1 reduction really doesn't suit, and 19 by 32 is not a sensible solution.

    For a reasonable guide that's not overly technical have a read of note 4 here : https://www.vicprop.com/propeller101.htm
     
  12. MikeJohns
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    MikeJohns Senior Member


    Any foil behind a prop reduces the tangential component of the prop wake and increases the inline thrust in consequence.
     
  13. yodani
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    yodani Senior Member

    Well I have inserted the data in the Vicprop calc and that's what it gave me.
    Boat water length 9m
    Beam at water line- 2.8m
    Draft- 1m
    Engine - 50hp@1800rpm
    Two bearings on schaft

    So it's clear this will not work. What reliable gearbox with 1.5 reduction ratio do you recommend? Commercial use grade.

    Cheers
     
  14. Barry
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    Barry Senior Member

    As with MR E, I am also a disbeliever of this comment

    If anything the prop wash coming into the rudder not in line with the direction of the boat will increase drag.

    Certainly, counter rotating props, ie inlet angle to the second prop is higher than the first prop helps. And so do vanes that change the direction of the moving water, but these vanes require an inlet angle offset from the keel line of the boat.

    If the rudder was a foil, there will be a slight pressure recovery from the higher pressure acting on the front of the foil providing recover to the trailing edge
     

  15. gonzo
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    gonzo Senior Member

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